1. Field of the Invention
The present invention relates to the testing of printed circuit boards. The invention has particular utility in connection with determined test fixtures of the type having test probes on a grid pattern, in which a translator pin fixture is used for translating electrical current from an off-grid pattern on a board under test to the channels of a tester in which the channel contacts are arranged in a grid pattern, and will be described in connection with such utility, although other utilities are contemplated. An especially particular utility for the present invention is found in the area of facilitating electrical testing of electrical connectors mounted on a circuit board (commonly referred to as "back plane" or "mid-plane"). These connectors typically are surrounded by a sidewall extending beyond the ends of the electrical contact posts.
2. Brief Description of Related Prior Art
Automatic test equipment for checking printed circuit boards has long involved the use of "bed of nails" test fixtures on which the circuit board is mounted during testing. A typical test fixture includes a large number of nail-like test probes arranged to make electrical contact between spring loaded contacts in the test equipment and designated test points on the circuit board under test, also referred to as the unit under test or "UUT". Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, arrangement of test probes for contacting test points on the board must be customized in a test fixture for that particular circuit board. Board design and fabrication data is used to determine what specific board features are to be tested by the fixture. A "determined" grid test fixture is typically fabricated by drilling patterns of holes in several rigid and non-conducting plates, e.g. of Lexan.RTM., assembling those plates with suitable fasteners and spacers to maintain said plates in a parallel, aligned position and then mounting test pins or probes in the drilled holes. Each plate has a hole pattern which is unique such that the test pin can only be inserted to provide an xy and z translation between a unique feature on the UUT and a unique tester grid channel. The circuit board is then positioned in the fixture precisely aligned with the array of test probes. During testing, the pins in the fixture are brought into spring-pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred between the board and the tester through the fixture so that a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board can perform the actual test.
Various approaches have been used in the past for bringing the test probes and the circuit board under test into pressure contact for testing. One class of these fixtures is a "wired" test fixture or a "dedicated" test fixture in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external electronically controlled test analyzer. These wired test fixtures are often referred to as "vacuum test fixtures" since a vacuum is applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. Customized wired test fixtures of similar construction also can be made by using mechanical means other than vacuum to apply the spring force necessary for compressing the board into contact with the probes during testing.
A further class of test fixtures is the so-called "grid-type fixture" test fixture, also known as a "determined" fixture, in which the random pattern of test points on the board are contacted by translator pins which transfer test signals to spring loaded interface pins arranged in a grid pattern in the tester. In these grid-type testers, fixturing is generally less complex and can be produced at lower cost than in the customized wired test fixtures; but with a grid system, the grid interfaces and test electronics are substantially more complex and costly.
A typical tester may have thousands of switches and channels. Each channel may have several switches, and is addressable and serves as one coordinate in the "grid". The tester has spring-loaded contacts which comprise the grid. The fixture contains rigid translator pins which conduct current from the grid channels to the UUT. In this way, the tester's computer can be made to test continuity and isolation in the UUT through the fixture. When testing a bare board on such a tester, a translator fixture supports and guides rigid pins that conduct between a grid pattern of spring-loaded probes in a grid base and an off-grid pattern of test points on the board under test. In one prior art grid fixture so-called "tilt pins" are used as the translator pins. The tilt pins are straight solid pins mounted in corresponding pre-drilled holes in translator plates which are part of the translator fixture. The tilt pins can tilt in various orientations to translate separate test signals from the off-grid random pattern of test points on the board to the grid pattern of test probes in the grid base.
Further details of prior art fixtures are found, for example, in U.S. Pat. No. 5,493,230 and U.S. Pat. No. 4,721,908.
It has been found that prior art testing apparatus of the types discussed above are not well suited for testing of contact posts mounted in connectors having sidewalls which extend beyond the ends of the contact posts. As a result, it often proved difficult to achieve good, stable electrical connection between the contact posts and the translator pins using prior art fixtures. Additionally, due to the tilting required to make contact to the uniform grid of the tester, the translator pins are rarely coaxial with the contact posts when they contact the posts. Since the contact posts usually are spaced very close together in the connector, this can cause translator pins to inadvertently become electrically connected to the wrong contact posts, and/or to each other. These can result in erroneous test results, damage to the circuit board, and/or damage to the testing apparatus. Moreover, with prior art fixtures, it was necessary to drill numerous layers of Lexan.RTM. to support and guide the rigid test probes to the level of the contact tips. Additionally, it was necessary to provide clearance slots on either side of the drilled holes for the connector side walls. While such techniques were able somewhat to reduce the risk of occurrences of inadvertent connection of translator pins to wrong contact posts, each other, and the sidewalls of the connectors, such drilling compromised the rigidity of the structure laterally, and often resulted in false opens and shorts during the testing process. Additionally, such techniques added to costs.
It is therefore an object of the present invention to provide a system for use in testing of printed circuit boards which overcomes the aforesaid and other objects of the present invention.
A more specific object of the invention is to provide an improved fixture and guide block assembly particularly for testing contact posts mounted in connectors having side walls that extend beyond the tips of the contact posts.